Calcium influx into the platelet controls important processes during platelet activation. Sage, S. O. Exp Physiol. (1997) 82:807–823; Mills, D. C. Thromb Haemost. (1996) 76:835–856. The pathway for this calcium entry is not well understood. Recently, gating of an ADP receptor (P2×) has been suggested to be responsible for rapid calcium (Ca2+) influx into the platelet. Mackenzie, A. B., et al., J Biol Chem. (1996) 271:2879–2881; Sun B., et al., J Biol Chem. (1998) 273:11544–11547. Indeed, several ions, including Ca2+, Mg2+ and Na+ enter when this receptor is activated. However, calcium entry via this receptor is believed not to be sufficient for the processes that take place during platelet activation. Jin, J. and Kanapuli S., Proc Natl Sci USA. (1998) 95:8070–8074; Kanapuli, S., Trends Pharmacol Sci. (1998) 19:391–394.
P2× is an ionotropic channel, and its gating in other cell types, such as skeletal muscle cells, has an excitatory depolarizing effect that activates voltage dependent calcium channels (VDCCs) (Bean, B. P., Trends Phys Sci. (1993) 13:87–90; Surprenant A., Trends Neurosci. (1995) 18:224–229; Abbrachio, M. P., et al., Pharmacol Ther. (1994) 64:445–475. It is the activation of these calcium selective VDCCs that triggers calcium dependent events inside these cells, such as secretion and contraction. Ashcroft, F. M., Ion channels and disease. San Diego, Calif.: Academic Press; (2000); Boyd, A. E., J Cell Biochem. (1992) 48:234–261; Hille B., Ionic channels of excitable membranes. Sunderland, Mass.: Sinauer Associate, Inc. publishers; (1992); Armstrong, C. M. and Hille, B., Neuron (1998) 20:371–380; Berridge, M. J., J Physiol (Lond). (1997) 499.2:291–306.
The presence of VDCCs in platelets is controversial (Sage, S. O., Exp Physiol. (1997) 82:807–823), and heretofor their expression has not been investigated at the molecular level. Thus, in view of the role of calcium influx in controlling processes during platelet activation, the identification of a VDCC polypeptide in platelets represents a long-felt and continuing need in the art.